Bias temperature instability of nano-scale silicon transistors

• Main Author: Ho, Terence Jun Jie
• Other Authors: A S Madhukumar
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Semiconductors
• Online Access: http://hdl.handle.net/10356/55442
• Institution: Nanyang Technological University

Negative Bias Temperature Instability (NBTI) has been a critical reliability issue for today’s sub-micron devices. It was experimentally noted that NBTI is a crucial limiting performance factor for PMOSFET. Thus, it defines that lifetime of the PMOSFET in the circuit. Under NBTI stress, the PMOSFET is subjected to a high electric field across its dielectric within a high temperature ambience. NBTI would result in deviation of device properties, such as threshold voltage and drain current, which are critical parameters when it comes to monitor device performance. In this project, the physics of NBTI is intensively studied. Conventional NBTI theories, such as the Reaction-Diffusion (R-D) model, have been used to explain NBTI. However, the author would provide evidence that this model would have to be extended, in order to provide a wider picture of NBTI. At the same time, dimensions (channel length) of the PMOSFET are being aggressively reduced so as to gain better device performance. Thus, the author would also take the opportunity to investigate the inter-relationship between decreasing channel lengths and NBTI physics. Apart from this inter-relationship, the role of nitrogen would also be investigated. It is common industrial practice to include nitrogen into the dielectric, so as to increase dielectric constant and prevent boron diffusion. However, earlier works have shown that implanting nitrogen into the dielectric worsens NBTI degradation. Thus, it provides the author a chance to understand how the presence of nitrogen affects the inter-relationship between decreasing channel length and NBTI physics. To further extend the work on NBTI, the author proceeds to investigate the inter-relationship between NBTI and Hot Carrier (HC) degradation. In practical circuits, the PMOSFET often gets subjected to both NBTI and HC stress at the same time. Thus, it would be appropriate to investigate their inter-relationship. Through experiments, the author was able to show the effect of HC stress on NBTI components (Recoverable and Permanent). The permanent component of NBTI was shown to increase gradually with NBTI stress time and had been attributed to the generation of dangling Si bonds at the interface. Similarly, the recoverable component of NBTI has shown independence of HC stress voltage and time. This concludes that both permanent and recoverable component are two distinct components, and the recoverable component is linked to switching traps located within the bulk of the dielectric. With this, the research work went on to discuss the impact of HC stress on deep and shallow recoverable component. Deep recoverable component are switching bulk traps located within the higher energy range and vice versa for shallow recoverable component. By adding HC stress, it was shown that shallow recoverable component can be converted to deep recoverable component, which results in an overall increase of the deep recoverable component. Taking on the conclusions of both earlier works, the author would then prove that there is no physical link between the interface and bulk trap, as claimed by other research groups. The last major part of this project focuses on the impact of ambience temperature on the inter-relationship between HC and NBTI degradation. By varying ambience temperature, the author would study how HC stress affects both components of NBTI. As expected, the permanent component shows a direct dependence on ambience temperature, which further strengthens its link to dangling Si bonds at the interface. As for the recoverable component, a novel finding was observed. At low ambience temperature, HC stress would result in a drop of the recoverable component. This is due to highly energetic phonons being able to reach the precursors of the recoverable component. However, as the ambience temperature increases, the effect of HC stress on the recoverable component is significantly reduced. This is explained by shorter free mean path of the phonons. At higher ambience temperature, the lattice vibrates at a higher frequency, due to higher energy being provided to the lattice atoms. When phonons are created as the result of HC stress, these phonons do interact with the lattice as they move away from the local hot spot. Due to higher vibrations, these phonons would quickly lose its energy and would not be able to reach any precursors located within the dielectric. As a result, at higher ambience temperature, it was observed that there are no effects of HC stress on the recoverable component of NBTI.